Flow-resistive Inspiratory Muscle Training Improves Running Time To Exhaustion With Thoracic Load Carriage

Abstract

Exercise while carrying an external load upon the thoracic cavity imposes extra stress on the cardiopulmonary and limb locomotor systems. This in turn, negatively impacts exercise tolerance and performance, as well as pulmonary and respiratory muscle function. Thoracic load carriage exercise (LC) has been shown to induce global respiratory muscle fatigue as assessed by volitional mouth pressures, with a concomitant impairment in running time-trial performance. Inspiratory muscle training (IMT) has been shown to improve performance in a running time-trial with thoracic LC. However, in many occupational and recreational activities that require thoracic LC, the capacity to sustain prolonged exercise may be of equal or greater importance than performance in a time-trial. PURPOSE: To determine the efficacy of 6 weeks of flow-resistive IMT on running time to exhaustion with thoracic LC; and to determine whether 6 weeks of flow-resistive IMT moderates diaphragmatic fatigue that may occur following a thoracic LC running time to exhaustion test. METHODS: Twelve recreationally active males completed two runs to exhaustion (Tlim) at a fixed speed eliciting 70 % of V˙O2max while carrying a 10 kg backpack. Visits were completed at baseline and after 6 weeks of either IMT or placebo-IMT. Exercise metabolic and ventilatory measures were recorded and diaphragm strength was measured using bilateral phrenic nerve stimulation in conjunction with esophageal balloon-tipped catheters to measure intrathoracic pressures. Twitch transdiaphragmatic pressure amplitude was recorded pre-exercise and immediately post-exercise in both trials. Maximal volitional mouth pressures (PImax) were recorded at baseline and post-IMT. RESULTS: 6 weeks of IMT significantly improved Tlim (p = 0.029, %Δ +29.3 ± 6.4 % IMT, -8.8 ± 11.1 %), but did not alter the change in diaphragm strength following a run to exhaustion (p > 0.05). The %Δ in PImax from pre- to post-training was significantly correlated with the %Δ in Tlim from pre- to post-training (p = 0.031, r = 0.622). No changes were observed in minute ventilation or breathing mechanics (all p > 0.05). CONCLUSIONS: IMT improves exercise tolerance with thoracic LC, but does not attenuate the severity of diaphragmatic fatigue following a running time to exhaustion test.